Primer

HTML

<div class="field-set field-set--merged">
    <span class="field-set__title" for="postcode">Text Field</span>

    <div class="field-set__content">
        <div class="field-set__items">
            
                            <div class="field-set__item form-large-col-4">
                    <label class="field-set__label">Text Field</label>
                    <input class="field-set__input" type="text">
                </div>
                            <div class="field-set__item form-large-col-4">
                    <label class="field-set__label">Text Field</label>
                    <input class="field-set__input" type="text">
                </div>
                            <div class="field-set__item form-large-col-4">
                    <label class="field-set__label">Text Field</label>
                    <input class="field-set__input" type="text">
                </div>
            
        </div>
    </div>
</div>

Twig

<div class="field-set field-set--merged">
    <span class="field-set__title" for="postcode">{{ merged_field_label }}</span>

    <div class="field-set__content">
        <div class="field-set__items">
            
            {% for field in merged_fields %}
                <div class="field-set__item form-large-col-{{ merge_fields_col }}">
                    <label class="field-set__label">Text Field</label>
                    <input class="field-set__input" type="text">
                </div>
            {% endfor %}

        </div>
    </div>
</div>

{
    "merged_field_label": "Text Field",
    "merge_fields_col": 4,
    "merged_fields": {
        "0": {
            "label": "Text Field"
        },
        "1": {
            "label": "Text Field"
        },
        "2": {
            "label": "Text Field"
        }
    }
}

Text Field